Re: [Qemu-devel] [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension

[Pavel Borzenkov]

On Wed, Mar 23, 2016 at 07:14:54PM +0100, Kevin Wolf wrote:
> Am 23.03.2016 um 18:58 hat Wouter Verhelst geschrieben:
> > On Wed, Mar 23, 2016 at 05:16:02PM +0300, Denis V. Lunev wrote:
> > > +    The type of information required by the client is passed to server 
> > > in the
> > > +    command flags field. If the server does not implement requested type 
> > > or
> > > +    have no means to express it, it MUST NOT return an error, but 
> > > instead MUST
> > > +    return a single LBA status descriptor with *offset* and *length* 
> > > equal to
> > > +    the *offset* and *length* from request, and *status* set to `0`.
> > > +
> > > +    The following request types are currently defined for the command:
> > > +
> > > +    1. Block provisioning state
> > > +
> > > +    Upon receiving an `NBD_CMD_GET_LBA_STATUS` command with command flags
> > > +    field set to `NBD_FLAG_GET_ALLOCATED` (0x0), the server MUST return
> > 
> > I prefer to have a non-zero flag value.
> > 
> > > +    the provisioning state of the device. The following provisionnig 
> > > states
> > > +    are defined for the command:
> > > +
> > > +      - `NBD_STATE_ALLOCATED` (0x0), LBA extent is present on the block 
> > > device;
> > > +      - `NBD_STATE_ZEROED` (0x1), LBA extent is present on the block 
> > > device
> > > +        and contains zeroes;
> > 
> > Presumably this should be "contains only zeroes"?
> > 
> > Also, this may end up being a fairly expensive call for the server to
> > process. Is it really useful?
> 
> I think we need to make clear that this is meant as an optimisation and
> it's always a valid option for a server to return NBD_STATE_ALLOCATED
> even if the contents is zeroed.
> 
> It is definitely useful if the server has a means to efficiently find
> out the allocation status (e.g. SEEK_HOLE). In that case the client may
> be able to avoid reading the block and sending it over the network, or
> when making a copy, it could use it to keep the target file sparse. If
> the client can't take advantage, we didn't have much overhead, so it's
> fine.

Yes, that was the idea. I'll add a note that the server may return
NBD_STATE_ALLOCATED instead of NBD_STATE_ZEROED if it has not means to
efficiently differentiate allocated blocks with zeroes from allocated
blocks with non-zeroed content.

> 
> It's less clear in a case where the server needs to read in the block
> and scan its contents. It could still be a net win if the next thing the
> client does is retrieving the block: The server would still have the
> cost of reading the block, but it wouldn't be transferred. But when the
> client doesn't follow up with a read, it's quite a bit of overhead that
> we had for no benefit. Returning NBD_STATE_ALLOCATED might be more
> appropriate in this case than scanning for zeros.
> 
> Kevin



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